We are working on the relationship of pituitary and serum growth hormone (GH) and prolactin (PRL) concentrations in relation to mouse mammary tumorigenesis in C3H strain, using the C57BL strain as a contrast because of its resistance. We use specific homologous radioimmunoassays for mouse GH and PRL. We are concerned with the natural history of secretory patterns by age, sex, nutritional state, and drug influence. We are concerned with the influence of stimuli to secretion and of suppressants on the physical state of the hormones in the pituitary gland, as evidenced by extractability, and on the processes by which these hormones are secreted.